The present invention relates in general to operational amplifiers and, more particularly, to an input stage of an operational amplifier operating rail-to-rail with a constant transconductance.
Operational amplifiers are commonly used in electronic circuit design to amplify an input signal and provide an amplified output signal. The operational amplifier typically includes an input stage for the initial amplification and an output stage for providing drive capability and possibly further amplification. The amplifier may further include intermediate stages for level shifting and conversion to single-ended operation.
The input stage to the operational amplifier is provides initial gain and is used to define the bandwidth of the amplifier. The input stage has an associated transconductance as the gain between the voltage input signal and the current supply to either the intermediate stages or the final output stage of the amplifier. It is desirable to have a constant transconductance for the input stage to provide constant bandwidth and constant gain for the amplifier. If the transconductance of the amplifier of the input stage varies, the output stage bandwidth must be increased to maintain stability for the amplifier.
One amplifier input stage known in the prior art includes complementary p-channel and n-channel differential transistor pairs. A single current source biases one pair of the differential amplifiers, for example the p-channel transistors. The current source is routed through a current mirror circuit to bias the opposite differential pair of the n-channel transistors. A current steering mechanism controls how much of the current source is applied to each differential transistor pair. Unfortunately, such a structure allows the common mode input voltage to vary the transconductance of the differential input pairs by 41% or more, over the full common mode input range. As noted above, the variation in transconductance requires a wider bandwidth in the output stage to prevent instability problems.
Hence, a need exists for an input stage of an operational amplifier operating rail-to-rail with a constant transconductance.